Large-scale consumption of traditional, fossil fuels (petroleum-based fuels) in recent decades has contributed to high levels of pollution. This, along with the realisation that the world stock of fossil fuels is not unlimited and a growing environmental awareness, has stimulated new initiatives to investigate the feasibility of alternative fuels such as ethanol, which is a particulate-free burning fuel source that releases less CO2 than unleaded gasoline on a per liter basis.
Although biomass-derived ethanol may be produced by the fermentation of hexose sugars obtained from many different sources, the substrates typically used for commercial scale production of fuel alcohol, such as cane sugar and corn starch, are expensive. Increases in the production of fuel ethanol will therefore require the use of lower-cost feedstocks.
Currently, only lignocellulosic feedstock derived from plant biomass is available in sufficient quantities to substitute the crops currently used for ethanol production. In most lignocellulosic material, the second-most-common sugar, after glucose, is xylose. Also L-arabinose is a sugar derived from some lignocellulosic material. Thus, for an economically feasible fuel production process, both hexose and pentose sugars must be fermented to form ethanol. The yeast Saccharomyces cerevisiae is robust and well adapted for ethanol production, but it is unable to produce ethanol using xylose as a carbon source. There is therefore a need for an organism possessing these properties so as to enable the commercially-viable production of ethanol from lignocellulosic feedstocks. Xylose isomerase from the anaerobic fungus Piromyces Sp.E2 was introduced in S. cerevisiae and high levels of enzyme activities were observed enabling this strain to grow anaerobically and produce ethanol from xylose (WO2003/062430 and WO06/009434). Such yeast strains for the first time provided specific rates of xylose consumption and ethanol formation that are compatible with ethanol production at a commercial scale.
However, it is still desirable to improve pentose conversion and to reduce fermentation time.